Method and apparatus for generating transmission signal by processing which includes digital quadrature modulation

ABSTRACT

Non-compensation signal points are determined in a two-dimensional plane without considering a signal error caused by digital quadrature modulation. The two-dimensional plane is defined by a real axis and an imaginary axis. The real axis corresponds to real-part signal components. The imaginary axis corresponds to imaginary-part signal components. Compensation signal points are determined in the two-dimensional plane in response to a signal error caused by digital quadrature modulation if the non-compensation signal points are used. The non-compensation signal points and the compensation signal points are point-symmetry. Digital information signal pieces are sequentially assigned to one of the compensation signal points in response to contents of the digital information signal pieces. The digital information pieces are subjected to a modulation process including digital quadrature modulation in response to the above-mentioned assignment to generate a radio-frequency transmission signal.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a method of generating a transmissionsignal by processing which includes digital quadrature modulation. Also,this invention relates to an apparatus for generating a transmissionsignal by processing which includes digital quadrature modulation.

[0003] 2. Description of the Related Art

[0004] Japanese patent application publication number 8-102766 disclosesdigital quadrature modulators in which an I-channel (in-phase channel)carrier is regarded as a repetitive data sequence of “1”→“0”→“−1”→“0”,and a Q-channel (quadrature channel) carrier is regarded as a repetitivedata sequence of “0”→“1”→“0”→“−1”. A digital I-channel informationsignal is sequentially multiplied by the I-channel carrier datasequence, while a digital Q-channel information signal is sequentiallymultiplied by the Q-channel carrier data sequence. A signal generated bythe multiplication in the I-channel and a signal generated by themultiplication in the Q channel are multiplexed into adigital-quadrature-modulation result signal.

[0005] In the case where samples of the digital I-channel informationsignal and samples of the digital Q-channel information signal aresynchronized with each other, the 90-degree (π/2) phase differencebetween the I-channel and Q-channel carrier data sequences causes atiming phase difference between the I-channel components and theQ-channel components of the digital-quadrature-modulation result signal.Such a timing phase difference adversely affects signal transmission.

[0006] Japanese application 8-102766 discloses that digital filters areprovided respectively in I-channel and Q-channel signal flow pathsbefore a stage for multiplexing the digital I-channel and Q-channelinformation signals by the I-channel and Q-channel carrier datasequences. The I-channel and Q-channel digital filters are designed toprovide different signal phases to compensate for the timing phasedifference between the I-channel components and the Q-channel componentsof the digital-quadrature-modulation result signal.

[0007] The I-channel and Q-channel digital filters in Japaneseapplication 8-102766 are required to implement accurate operation. Inaddition, the I-channel and Q-channel digital filters have complicatedstructures, and are hence expensive.

SUMMARY OF THE INVENTION

[0008] It is a first object of this invention to provide an improvedmethod of generating a transmission signal by processing which includesdigital quadrature modulation.

[0009] It is a second object of this invention to provide an improvedapparatus for generating a transmission signal by processing whichincludes digital quadrature modulation.

[0010] A first aspect of this invention provides a method of generatinga transmission signal. The method comprises the steps of determiningnon-compensation signal points in a two-dimensional plane withoutconsidering a signal error caused by digital quadrature modulation, thetwo-dimensional plane being defined by a real axis and an imaginaryaxis, the real axis corresponding to real-part signal components, theimaginary axis corresponding to imaginary-part signal components;determining compensation signal points in the two-dimensional plane inresponse to a signal error caused by digital quadrature modulation ifthe non-compensation signal points are used, the non-compensation signalpoints and the compensation signal points being point-symmetry;sequentially assigning digital information signal pieces to one of thecompensation signal points in response to contents of the digitalinformation signal pieces; and subjecting the digital information piecesto a modulation process including digital quadrature modulation inresponse to the assignment given by the assigning step to generate aradio-frequency transmission signal.

[0011] A second aspect of this invention is based on the first aspectthereof, and provides a method wherein the compensation signal pointsprovide compensation for an error in the radio-frequency transmissionsignal which is caused by one of a phase difference between an in-phasesignal and a quadrature signal, an amplitude difference between thein-phase signal and the quadrature signal, and an error in a quadraturerelation between the in-phase signal and the quadrature signal.

[0012] A third aspect of this invention is based on the first aspectthereof, and provides a method wherein the compensation signal pointsprovide compensation for an error in the radio-frequency transmissionsignal which is caused by a timing difference between an in-phase signaland a quadrature signal.

[0013] A fourth aspect of this invention provides a method of generatinga transmission signal. The method comprises the steps of determiningfirst non-compensation signal points in a two-dimensional plane withoutconsidering a signal error caused by digital quadrature modulation, thetwo-dimensional plane being defined by a real axis and an imaginaryaxis, the real axis corresponding to real-part signal components, theimaginary axis corresponding to imaginary-part signal components;determining second non-compensation signal points in the two-dimensionalplane without considering the signal error caused by digital quadraturemodulation; determining first compensation signal points in thetwo-dimensional plane for a first subcarrier in response to a signalerror caused by digital quadrature modulation if the firstnon-compensation signal points are used, the first non-compensationsignal points and the first compensation signal points beingpoint-symmetry; determining second compensation signal points in thetwo-dimensional plane for a second subcarrier in response to a signalerror caused by digital quadrature modulation if the secondnon-compensation signal points are used, the second non-compensationsignal points and the second compensation signal points beingpoint-symmetry, the second subcarrier being equal in frequency to thefirst subcarrier and being different in polarity from the firstsubcarrier; sequentially assigning first digital information signalpieces to one of the first compensation signal points in response tocontents of the first digital information signal pieces; sequentiallyassigning second digital information signal pieces to one of the secondcompensation signal points in response to contents of the second digitalinformation signal pieces; and subjecting the first digital informationpieces and the second digital information pieces to a modulation processincluding digital quadrature modulation in response to the assignmentsgiven by the assigning steps to generate a radio-frequency transmissionsignal containing the first and second subcarriers.

[0014] A fifth aspect of this invention provides an apparatus forgenerating a transmission signal. The apparatus comprises means fordetermining non-compensation signal points in a two-dimensional planewithout considering a signal error caused by digital quadraturemodulation, the two-dimensional plane being defined by a real axis andan imaginary axis, the real axis corresponding to real-part signalcomponents, the imaginary axis corresponding to imaginary-part signalcomponents; means for determining compensation signal points in thetwo-dimensional plane in response to a signal error caused by digitalquadrature modulation if the non-compensation signal points are used,the non-compensation signal points and the compensation signal pointsbeing point-symmetry; means for sequentially assigning digitalinformation signal pieces to one of the compensation signal points inresponse to contents of the digital information signal pieces; and meansfor subjecting the digital information pieces to a modulation processincluding digital quadrature modulation in response to the assignmentgiven by the assigning means to generate a radio-frequency transmissionsignal.

[0015] A sixth aspect of this invention provides an apparatus forgenerating a transmission signal. The apparatus comprises first meansfor storing information representing assignment of states of a signalpiece to signal points predetermined in response to an estimated signalerror caused by digital quadrature modulation in the absence ofcorrection; second means for assigning an input information signal pieceto one of the signal points in response to a state of the inputinformation signal piece according to the information stored in thefirst means to convert the input information signal piece into first andsecond baseband signal pieces; and third means for subjecting the firstand second baseband signal pieces generated by the second means to amodulation process including digital quadrature modulation to convertthe first and second baseband signal pieces into a piece of amodulation-resultant transmission signal from which a signal errorcaused by the digital quadrature modulation is removed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a block diagram of an OFDM (orthogonal frequencydivision multiplexing) modulation apparatus according to an embodimentof this invention.

[0017]FIG. 2 is a time-domain diagram of a first example of signals inthe apparatus of FIG. 1.

[0018]FIG. 3 is a time-domain diagram of a second example of signals inthe apparatus of FIG. 1.

[0019]FIG. 4 is a diagram of a complex plane in which a state of asubcarrier is illustrated.

[0020]FIG. 5 is a diagram of a complex plane in which vectorscorresponding to a subcarrier are illustrated.

[0021]FIG. 6 is a diagram of a complex plane in which vectorscorresponding to a subcarrier are illustrated.

[0022]FIG. 7 is a diagram of a complex plane in which vectorscorresponding to a subcarrier are illustrated.

[0023]FIG. 8 is a diagram of a complex plane in which vectorscorresponding to a subcarrier are illustrated.

[0024]FIG. 9 is a diagram of a complex plane in which vectorscorresponding to a subcarrier are illustrated.

[0025]FIG. 10 is a diagram of a complex plane in which vectorscorresponding to a subcarrier are illustrated.

DETAILED DESCRIPTION OF THE INVENTION

[0026]FIG. 1 shows an OFDM (orthogonal frequency division multiplexing)modulation apparatus according to an embodiment of this invention.

[0027] The apparatus of FIG. 1 includes a data mapping circuit 11, amapping table 12, an IFFT (inverse fast Fourier transform) device 13, adigital quadrature modulator 15, an intermediate-frequency oscillator16, and a D/A (digital-to-analog) converter 17. The data mapping circuit11, the IFFT device 13, the digital quadrature modulator 15, and the D/Aconverter 17 are sequentially connected in that order. The mapping table12 is provided in a memory connected to the data mapping circuit 11. Theintermediate-frequency oscillator 16 is connected to the digitalquadrature modulator 15.

[0028] The data mapping circuit 11 receives input digital data to betransmitted. The data mapping circuit 11 converts the input digital datainto digital I (in-phase) signals “i” and digital Q (quadrature) signals“q” in response to the contents of the mapping table 12. Specifically,the data mapping circuit 11 cyclically assigns successive segments ofthe input digital data to pairs of subcarriers of an OFDM signal,respectively. The mapping table 12 contains information indicatingsignal points of QPSK-resultant (quadrature phase shift keyingresultant) subcarriers, QAM-resultant (quadrature amplitude modulationresultant) subcarriers, or BPSK-resultant (binary phase shift keyingresultant) subcarriers. The signal points are defined in anamplitude-angle plane. As will be made clear later, the signal pointsare predetermined so as to compensate for a signal error caused by thequadrature modulator 15. The mapping table 12 also contains informationindicating each assignment of values which can be represented by asegment of the input digital data to signal points of a correspondingpair of QPSK-resultant subcarriers, QAM-resultant subcarriers, orBPSK-resultant subcarriers. The data mapping circuit 11 implements datamapping on the segments of the input digital data in accordance with thecontents of the mapping table as follows. Specifically, the data mappingcircuit 11 assigns every segment of the input digital data to one ofcorresponding signal points in response to the value represented by thesegment of the input digital data by referring to the contents of themapping table 12. In other words, for every segment of the input digitaldata, the data mapping circuit 11 selects one from among correspondingsignal points in response to the value represented by the segment of theinput digital data. The data mapping circuit 11 generates the digitalsignals “i” and the digital signals “q” on the basis of theamplitude-direction coordinates and the angle-direction coordinates ofthe assigned signal points (the selected signal points). The datamapping circuit 11 feeds the digital signals “i” and the digital signals“q” to the IFFT device 13. The digital signals “i” and the digitalsignals “q” are arranged in a virtual frequency domain so that they canbe properly assigned to frequencies (orthogonal baseband subcarrierfrequencies) for IFFT.

[0029] The IFFT device 13 implements IFFT (inverse fast Fouriertransform) while setting the digital signals “i” as real-part terms andsetting the digital signals “q” as imaginary-part terms. The IFFT device13 converts and combines the digital signals “i” into an IFFT-resultantdigital I signal or an IFFT-resultant digital real-part signal. Inaddition, the IFFT device 13 converts and combines the digital signals“q” into an IFFT-resultant digital Q signal or an IFFT-resultant digitalimaginary-part signal. The IFFT device 13 outputs the IFFT-resultantdigital I and Q signals to the digital quadrature modulator 15. The IFFTdevice 13 includes an n-point IFFT circuit, where “n” denotes apredetermined natural number.

[0030] The digital quadrature modulator 15 includes multipliers 15A,15B, 15C, and 15D, and a data selector 15E. The multipliers 15A, 15B,15C, and 15D follow the IFFT device 13. The data selector 15E followsthe multipliers 15A, 15B, 15C, and 15D. The data selector 15E isconnected to the intermediate-frequency oscillator 16 and the D/Aconverter 17.

[0031] The multiplier 15A receives the IFFT-resultant digital I signalfrom the IFFT device 13, and multiplies the IFFT-resultant digital Isignal by “1”. The multiplier 15A includes a buffer or an amplifierhaving a gain of 1. The multiplier 15A outputs themultiplication-resultant digital signal “I” to the data selector 15E.The multiplier 15B receives the IFFT-resultant digital Q signal from theIFFT device 13, and multiplies the IFFT-resultant digital Q signal by“−1”. The multiplier 15B includes an inverter. The multiplier 15Boutputs the multiplication-resultant digital signal “−Q” to the dataselector 15E. The multiplier 15C receives the IFFT-resultant digital Isignal from the IFFT device 13, and multiplies the IFFT-resultantdigital I signal by “−1”. The multiplier 15C includes an inverter. Themultiplier 15C outputs the multiplication-resultant digital signal “−I”to the data selector 15E. The multiplier 15D receives the IFFT-resultantdigital Q signal from the IFFT device 13, and multiplies theIFFT-resultant digital Q signal by “1”. The multiplier 15D includes abuffer or an amplifier having a gain of 1. The multiplier 15D outputsthe multiplication-resultant digital signal “Q” to the data selector15E.

[0032] The intermediate-frequency oscillator 16 acts as a localoscillator. The intermediate-frequency oscillator 16 includes a clocksignal generator for producing a clock signal having a predeterminedfrequency. Preferably, the frequency of the clock signal is equal to apredetermined integer multiple of (for example, four times) thefrequency of virtual analog sine and cosine waves for quadraturemodulation. The intermediate-frequency oscillator 16 feeds the clocksignal to the data selector 15E.

[0033] The data selector 15E sequentially and cyclically selects onefrom among the output digital signals of the multipliers 15A, 15B, 15C,and 15D in response to the clock signal fed from theintermediate-frequency oscillator 16. The selection order in every cycleis as follows: “I” (the output digital signal of the multiplier 15A),“−Q” (the output digital signal of the multiplier 15B), “−I” (the outputdigital signal of the multiplier 15C), and “Q” (the output digitalsignal of the multiplier 15D). The data selector 15E outputs theselection-resultant digital signal to the D/A converter 17.

[0034] The multipliers 15A-15D and the data selector 15E cooperate toimplement digital quadrature modulation which combines theIFFT-resultant digital I and Q signals into a singlequadrature-modulation-resultant digital signal (the output digitalsignal from the data selector 15E). The digital quadrature modulationvirtually uses a digital sine wave and a digital cosine wave having afrequency corresponding to the cycle of the signal selection by the dataselector 15E. The digital sine wave periodically changes as“0”→“1”→“0”→“−1”. The digital cosine wave periodically changes as“1”→“0”→“−1”→“0”. The digital sine wave results from periodicallysampling the virtual analog sine wave at a prescribed frequency. Thedigital cosine wave results from periodically sampling the virtualanalog cosine wave at the prescribed frequency. The multiplications bythe multipliers 15A-15D correspond to the multiplications among theIFFT-resultant digital I and Q signals, the digital sine wave, and thedigital cosine wave for the digital quadrature modulation. For everycycle, the data selector 15E sequentially selects the output digitalsignals from the multipliers 15A-15D in the order of “I→Q→−I→Q”.

[0035] The D/A converter 17 changes the output digital signal of thedigital quadrature modulator 15, that is, the output digital signal ofthe data selector 15E, into a corresponding analog multiple-carriersignal (a corresponding analog OFDM signal) whose center frequency isequal to the frequency of the sine and cosine waves used in the digitalquadrature modulator 15. The D/A converter 15 outputs the analogmultiple-carrier signal.

[0036] The contents of the mapping table 12 include corrective data forpreviously correcting the error of the actual characteristics of thedigital quadrature modulator 15 from the desired characteristics. Thedata mapping by the data mapping circuit 11 in responsive to thecorrective-data-added mapping table 12 means that non-correctedmapping-resultant signals are revised in response to the correctivedata. The revision-resultant signals are used for generating the digitalsignals “i” and the digital signals “q”.

[0037] The analog multiple-carrier signal (the analog OFDM signal)outputted from the D/A converter 15 is fed via a frequency up-conversionstage to a receiving apparatus as a transmission signal while beingpropagated along a transmission line. The receiving apparatusdemodulates the analog multiple-carrier signal into signal-pointinformation pieces. The receiving apparatus recovers the originaldigital data from the signal-point information pieces.

[0038] As shown in FIG. 2, the IFFT-resultant digital I signal outputtedfrom the IFFT device 13 has a sequence of time-domain variable samples .. . , I_(n−1), I_(n), I_(n+1), . . . . Similarly, the IFFT-resultantdigital Q signal outputted from the IFFT device 13 has a sequence oftime-domain variable samples . . . , Q_(n−1), Q_(n), Q_(n+1), . . . .The period of samples of the IFFT-resultant digital I signal and theperiod of samples of the IFFT-resultant digital I signal correspond to asampling frequency related to the OFDM signal. The sample period has agiven relation with the window interval for operating the n-point IFFTcircuit in the IFFY device 13. Specifically, the sample periodcorresponds to the window interval divided by “n”.

[0039] During every sample period, the data selector 15E sequentiallyand cyclically selects “I_(n)” (the output digital signal of themultiplier 15A), “0”, “−I_(n)” (the output digital signal of themultiplier 15C), and “0”. Thus, regarding the IFFT-resultant digital Isignal, the data selector 15E generates the recurrence of “I_(n)”, “0”,“−I_(n)”, and “0”. During every sample period, the data selector 15Esequentially and cyclically selects “0”, “−Q_(n)” (the output digitalsignal of the multiplier 15B)”, “0”, and “Q_(n)” (the output digitalsignal of the multiplier 15D). Thus, regarding the IFFT-resultantdigital Q signal, the data selector 15E generates the recurrence of “0”,“−Q_(n)”, “0”, and “Q_(n)”. The data selector 15E combines therecurrence of “I_(n)”, “0”, “−I_(n)”, and “0” and the recurrence of “0”,“−Q_(n)”, “0”, and “Q_(n)” into the recurrence of “I_(n)”, “−Q_(n)”,“−I_(n)”, and “Qn”. The data selector 15E outputs the recurrence of“I_(n)”, “−Q_(n)”, “−I_(n)”, and “Qn” as a modulation-resultant digitalsignal.

[0040] With reference to FIG. 2, at the start of every sample period,the real-part component “I_(n)” of the modulation-resultant digitalsignal (that is, the output signal from the data selector 15E) precedesthe imaginary-part component “−Qn” thereof. Accordingly, there would bea timing difference or a phase difference between the real-partcomponents and the imaginary-part components of the modulation-resultantdigital signal.

[0041] The data mapping circuit 11 and the mapping table 12 (see FIG. 1)are designed to compensate for such a timing difference (a phasedifference). Specifically, the mapping table 12 contains data mappinginformation predetermined in response to an estimated timing differencebetween the real-part components and the imaginary-part components ofthe modulation-resultant digital signal generated by the digitalquadrature modulator 15 in the absence of correction or compensation.The data mapping circuit 11 accesses the mapping table 12, and convertsthe input digital data into the digital signals “i” and the digitalsignals “q” by referring to the contents of the mapping table 12 (thatis, the data mapping information in the mapping table 12). The digitalsignal “i” and the digital signal “q” provide compensation for theactual phase difference between the real-part components and theimaginary-part components of the modulation-resultant digital signalgenerated by the digital quadrature modulator 15. The data mappingcircuit 11 feeds the digital signal “i” and the digital signal “q” tothe IFFT device 13.

[0042] The timing difference or the phase difference between thereal-part components and the imaginary-part components of themodulation-resultant digital signal generated by the digital quadraturemodulator 15 corresponds to a timing error between the virtual sine waveand the virtual cosine wave used in the digital quadrature modulator 15,that is, an error in the quadrature relation between I and Q signalvectors with respect to the modulation-resultant digital signal. Themodulation-resultant digital signal has components (subcarriers) inupper and lower side bands of the intermediate frequency set by theintermediate-frequency oscillator 16. There are pairs of theupper-side-band signal components and the lower-side-band signalcomponents. The upper-side-band signal component and the lower-side-bandsignal component in each pair (that is, the upper-side-band subcarrierand the lower-side-band subcarrier in each pair) are located atrespective frequency points equidistant from the intermediate frequency.The above-indicated error in the quadrature relation would causecrosstalk between the upper-side-band signal component and thelower-side-band signal component in each pair. The crosstalk wouldadversely alter the frequency spectrum of the modulation-resultantdigital signal.

[0043] The data mapping circuit 11 and the mapping table 12 implementdata mapping which is designed to compensate for such crosstalk.Specifically, the data mapping is designed to correct theupper-side-band subcarrier and the lower-side-band subcarrier in eachpair.

[0044] The data mapping will be described below in more detail. Signalpoints are expressed by the coordinates in a complex plane. The datamapping uses signal points resulting from correction of original signalpoints. For example, an original signal point at the coordinates of(1, 1) is corrected into a second signal point at the coordinates of(1+x, 1+y) where “x” and “y” denote real-part and imaginary-partcorrective quantities respectively.

[0045] The upper-side-band subcarrier and the lower-side-band subcarrierin each pair are defined as a positive-frequency subcarrier and anegative-frequency subcarrier, respectively. For the positive-frequencysubcarrier and the negative-frequency subcarrier in each pair, thecorresponding before-correction signal point (the original signal point)is expressed by the values d1, d2, d3, and d4 which denote the real-partvalue for the positive-frequency subcarrier, the imaginary-part valuefor the positive-frequency subcarrier, the real-part value for thenegative-frequency subcarrier, and the imaginary-part value for thenegative-frequency subcarrier, respectively. For example, in the case ofQPSK, each of the values d1, d2, d3, and d4 assumes “+1” or “−1”. Thedata mapping reflects correction of the values d1, d2, d3, and d4 intothe values d1+x, d2+y, d3+x′, and d4+y′, respectively. Here, x, y, x′,and y′ denote corrective quantities or corrective values.

[0046]FIG. 3 shows an example of time-domain states of theIFFT-resultant digital I signal outputted from the IFFT device 13, theIFFT-resultant digital Q signal outputted from the IFFT device 13, theresult of the I signal selection by the data selector 15E, the result ofthe Q signal selection by the data selector 15E, and themodulation-resultant digital signal outputted from the data selector 15Ewhich occur under conditions as follows. The sample period of theIFFT-resultant digital I and Q signals is equal to 19.5 nsec. Thus, thesample frequency of the IFFT-resultant digital I and Q signals is equalto 51.2 MHz. The period of the signal selection by the data selector 15Eis equal to a half of the sample period of the IFFT-resultant digital Iand Q signals. Thus, the sample frequency of the modulation-resultantdigital signal is equal to 102.4 MHz. The intermediate frequency set bythe intermediate-frequency oscillator 16 is equal to 25.6 MHz.

[0047] With reference to FIG. 3, the IFFT-resultant digital I signal hasa sequence of time-domain variable samples . . . , I_(n−1), I_(n),I_(n+1), . . . . Similarly, the IFFT-resultant digital Q has a sequenceof time-domain variable samples . . . , Q_(n−1), Q_(n), Q_(n+1), . . . .Each of the samples of the IFFT-resultant digital I and Q signals has adirect-current component (a zero-frequency component) andalternating-current components having frequencies up to 16 MHz. Thedigital quadrature modulator 15 handles the IFFT-resultant digital I andQ signals as baseband signals. The digital quadrature modulator 15converts the IFFT-resultant digital I and Q signals into amodulation-resultant digital signal of an intermediate-frequency bandcentered at 25.6 MHz. Specifically, the modulation-resultant digitalsignal extends over a frequency band of 25.6±16 MHz. In the absence ofthe correction by the data mapping circuit 11 and the mapping table 12,the quadrature relation between I and Q signal vectors with respect tothe modulation-resultant digital signal would have an error of about 9.8nsec (1/102.4 MHz). A pair of the 38.4-MHz subcarrier and the 12.8-MHzsubcarrier which is higher and lower than the center frequency (25.6MHz) by 12.8 MHz is taken as an example. The quadrature-relation errorwould cause the 38.4-MHz subcarrier to have a phase delay of π/4 radian.On the other hand, the quadrature-relation error would cause the12.8-MHz subcarrier to have a phase advance of π/4 radian. Thepreviously-mentioned corrective quantities used in the data mappingcompensate for such phase delays and phase advances in the subcarriers.

[0048] The corrective quantities (the corrective values) are expressedin a two-dimensional plane defined by the real axis and the imaginaryaxis. FIG. 4 illustrates a two-dimensional plane (a complex plane)defined by the real axis and the imaginary axis. In FIG. 4, there isshown a state of a subcarrier having a phase angle “α”, an angularvelocity “+ω_(n)”, and an amplitude “A”. The subcarrier is expressed bythe following equation.

A·cos (+ω_(n) t+α)+j·A·sin (+ω_(n) t+α)  (1)

[0049] where “j” denotes an imaginary unit. For example, in the case ofQPSK, the amplitude “A” is equal to {square root}{square root over (2)},and the phase angle “α” assumes one of π/4, 3π/4, 5π/4, and 7π/4.

[0050] Similarly, a subcarrier having a phase angle “β”, an angularvelocity “−ω_(n)”, and an amplitude “B” is expressed by the followingequation.

B·cos (−ω_(n) t+β)+j·B·sin (−ω_(n) t+β)  (2)

[0051] For example, in the case of QPSK, the amplitude “B” is equal to{square root}{square root over (2)}, and the phase angle “β” assumes oneof π/4, 3π/4, 5π/4, and 7π/4. The subcarrier having the angular velocity“+ω_(n)” and the subcarrier having the angular velocity “−ω_(n)” form apair.

[0052] It is assumed that imaginary-part signals corresponding to theimaginary terms of the equations (1) and (2) have errors in amplitudeand phase relative to real-part signals corresponding to the real termsof the equations (1) and (2). Specifically, the assumed amplitude erroris equal to “λ” times while the assumed phase error is equal to “γ”radian. The error-containing subcarriers are expressed by the followingequations.

A·cos (+ω_(n) t+α)+j·λ·A·sin (+ω_(n) t+α−γ)  (3)

B·cos (−ω_(n) t+β)+j·λ·B·sin (−ω_(n) t+β+γ)  (4)

[0053] The phase error “γ” corresponds to the previously-mentioned errorequal to about 9.8 nsec, that is, the quadrature-relation error. As willbe explained later, the data mapping compensates for the phase error“γ”. In addition, the data mapping may compensate for the amplitudeerror “λ”.

[0054] By using an exponential function, the equation (1) is rewrittenas follows.

(a+j·b)·e ^(jω) n ^(t)  (5)

[0055] where a=A·cos (α), and b=A·sin (α). The equation (3) is rewrittenas follows.

(½)Ae ^(jα) e ^(jω) n ^(t)+(½)Ae ^(−jα) e ^(−jω) n ^(t)+λ(½)Ae ^(j(α−γ))e ^(jω) n ^(t)−λ(½)Ae ^(−j(α−γ)) e ^(−jω) n ^(t)  (6)

[0056] Similarly, the equation (4) is rewritten as follows.

(½)Be ^(jβ) e ^(−jω) n ^(t)+(½)Be ^(−jβ) e ^(jω) n ^(t)+λ(½)Be ^(j(β+γ))e ^(−jω) n ^(t)−λ(½)Be ^(−j(β+γ)) e ^(jω) n ^(t)  (7)

[0057] The first and third terms in the equation (6) denote vectors 61and 63 rotating at the angular velocity “ω_(n)t” which are shown in FIG.5. The first-term vector 61 in FIG. 5 has an amplitude “A/2” and anangle “α” from the real axis. The third-term vector 63 in FIG. 5 has anamplitude “λ·A/2” and an angle “α−γ” from the real axis.

[0058] The second and fourth terms in the equation (6) denote vectors 62and 64 rotating at the angular velocity “ω_(n)t” which are shown in FIG.6. The second-term vector 62 in FIG. 6 has an amplitude “A/2” and anangle “−α” from the real axis. The fourth-term vector 64 in FIG. 6 hasan amplitude “λ·A/2” and an angle “−(α−γ)” from the real axis. Thefourth-term vector 64 exists in the second quadrant of thetwo-dimensional plane.

[0059] The first-term vector 61 in FIG. 5 and the second-term vector 62in FIG. 6 correspond to the real-part signal expressed by the first termof the equation (1), while the third-term vector 63 in FIG. 5 and thefourth-term vector 64 in FIG. 6 correspond to the imaginary-part signalexpressed by the second term of the equation (1).

[0060] The first and third terms in the equation (7) denote vectors 71and 73 rotating at the angular velocity “−ω_(n)t” which are shown inFIG. 7. The first-term vector 71 in FIG. 7 has an amplitude “B/2” and anangle “β” from the real axis. The third-term vector 73 in FIG. 7 has anamplitude “λ·B/2” and an angle “β+γ” from the real axis.

[0061] The second and fourth terms in the equation (7) denote vectors 72and 74 rotating at the angular velocity “ω_(n)t” which are shown in FIG.8. The second-term vector 72 in FIG. 8 has an amplitude “B/2” and anangle “−β” from the real axis. The fourth-term vector 74 in FIG. 8 hasan amplitude “λ·B/2” and an angle “−(β+γ)” from the real axis. Thefourth-term vector 74 exists in the second quadrant of thetwo-dimensional plane.

[0062] The first-term vector 71 in FIG. 7 and the second-term vector 72in FIG. 8 correspond to the real-part signal expressed by the first termof the equation (2), while the third-term vector 73 in FIG. 7 and thefourth-term vector 74 in FIG. 8 correspond to the imaginary-part signalexpressed by the second term of the equation (2).

[0063] The previously-mentioned amplitude error (“λ” differs from “1”)and phase error (“γ” differs from “0”) relate to the third and fourthterms in the equations (6) and (7). The data mapping is designed tocompensate for the amplitude error “λ” and the phase error “γ”.Specifically, in the case where a corrective signal having an amplitudeof a factor of “1/λ” and a phase advance of “γ” is given with respect tothe third and fourth terms in the equation (6), the digital quadraturemodulator 15 multiplies the amplitude by “λ” and delays the phase by “γ”so that a signal having a nullified amplitude error and a nullifiedphase error will be generated regarding the third and fourth terms ofthe equation (6).

[0064] The corrective signal for the signal represented by the equation(6) is expressed as follows.

(½)Ae ^(jα) e ^(jω) n ^(t)+(½)Ae ^(−jα) e ^(−jω) n ^(t)+{1/(2λ)}Ae^(j(α+γ)) e ^(jω) n ^(t)−{1/(2λ)}Ae ^(−j(α+γ)) e ^(−jω) n ^(t)  (8)

[0065] Similarly, a corrective signal for the signal represented by theequation (7) is expressed as follows.

(½)Be ^(jβ) e ^(−jω) n ^(t)+(½)Be ^(−jβ) e ^(jω) n ^(t){1/(2λ)}Be^(j(β−γ)) e ^(−jω) n ^(t)−{1/(2λ)}Be ^(−j(β−γ)) e ^(jω) n ^(t)  (9)

[0066] The corrective signals expressed by the equations (8) and (9)correspond to the cancel of the amplitude error “λ” and the phase error“γ” from the before-correction signals given by the equations (3) and(4). Accordingly, it is possible to generate error-freemodulation-resultant digital signals expressed by the equations (1) and(2).

[0067]FIG. 9 shows a mapping point (a signal point) for enabling thedigital quadrature modulator 15 to generate an error-compensatedmodulation-resultant digital signal having the angular velocity“ω_(n)t”. In FIG. 9, vectors 81 and 83 correspond to the first and thirdterms of the equation (8) while vectors 92 and 94 correspond to thesecond and fourth terms of the equation (9). A final vector 101representing a correction-resultant signal point is calculated bysumming the vectors 81, 83, 92, and 94. The vectors 81 and 83 correspondto the terms containing “e^(jω)n^(t)”. Also, the vectors 92 and 94correspond to the terms containing “e^(jω)n^(t)”. Accordingly, the finalvector 101 is equal to the resultant of the vectors 81, 83, 92, and 94which rotate at the angular velocity “ω_(n)t”. Specifically, the finalvector 101 is calculated by summing the resultant of the vectors 81 and92 and the resultant of the vectors 83 and 94. The correction-resultantsignal point represented by the final vector 101 is assigned to thesubcarrier which rotates at the angular velocity “ω_(n)t”.

[0068]FIG. 10 shows a mapping point (a signal point) for enabling thedigital quadrature modulator 15 to generate an error-compensatedmodulation-resultant digital signal having the angular velocity“−ω_(n)t”. In FIG. 10, vectors 82 and 84 correspond to the second andfourth terms of the equation (8) while vectors 91 and 93 correspond tothe first and third terms of the equation (9). A final vector 102representing a correction-resultant signal point is calculated bysumming the vectors 82, 84, 91, and 93. The vectors 82 and 84 correspondto the terms containing “e^(−jω)n^(t)”. Also, the vectors 91 and 93correspond to the terms containing “e^(−jω)n^(t)”. Accordingly, thefinal vector 102 is equal to the resultant of the vectors 82, 84, 91,and 93 which rotate at the angular velocity “−ω_(n)t”. Thecorrection-resultant signal point represented by the final vector 102 isassigned to the subcarrier which rotates at the angular velocity“−ω_(n)t”.

[0069] As understood from the previous description, in order tocompensate for the phase error and the amplitude error caused by thedigital quadrature modulator 15 to generate the error-free subcarriers“A·cos (+ω_(n)t+α)+j·A·sin (+ω_(n)t+α)” and “B·cos (−ω_(n)t+β)+j·B·sin(−ω_(n)t+β)”, the signal points used by the data mapping are chosen tocorrespond to subcarriers “A·cos (+ω_(n)t+α)+j·(1/λ)·A·sin(+ω_(n)t+α+γ)” and “B·cos (−ω_(n)t+β)+j·(1/λ)·B·sin (−ω_(n)t+β−γ)”.

[0070] The sum of the coefficients in the terms of the equations (8) and(9) which relate to the subcarrier having the angular frequency “+ω_(n)”is given as follows.

(½)Ae ^(jα)+(½)Be ^(−jβ)+{1/(2λ)}Ae ^(j(α+γ))−{1/(2λ)}Be^(−j(β−γ))  (10)

[0071] The equation (10) is rewritten as follows.

Re(+ω_(n))+jIm(+ω_(n))  (11)

[0072] where:

Re(+ω_(n))=(½)A cos α+(½)B cos β+{1/(2λ)}A cos (α+γ)−{1/(2λ)}B cos(β−γ)  (12)

Im(+ω_(n))=(½)A sin α−(½)B sin β+{1/(2λ)}A sin (α+γ)+{1/(2λ)}B sin(β−γ)  (13)

[0073] The value Re(+ω_(n)) denoted by the equation (12) is assigned tothe real part for the subcarrier having the angular frequency “+ω_(n)”.The value Im(+ω_(n)) denoted by the equation (13) is assigned to theimaginary part for the subcarrier having the angular frequency “+ω_(n)”.

[0074] The sum of the coefficients in the terms of the equations (8) and(9) which relate to the subcarrier having the angular frequency “−ω_(n)”is given as follows.

(½)Be ^(jβ)+(½)Ae ^(−jα)+{1/(2λ)}Be ^(j(β−γ))−{1/(2λ)}Ae^(−j(α+γ))  (14)

[0075] The equation (13) is rewritten as follows.

Re(−ω_(n))+jIm(−ω_(n))  (15)

[0076] where:

[0077]  Re(−ω_(n))=(½)B cos β+(½)A cos α+{1/(2λ)}B cos (β−γ)−{1/(2λ)}Acos (α+γ)  (16)

Im(−ω_(n))=(½)B sin β−(½)A sin α+{1/(2λ)}B sin (β−γ)+{1/(2λ)}A sin(α+γ)  (17)

[0078] The value Re(−ω_(n)) denoted by the equation (16) is assigned tothe real part for the subcarrier having the angular frequency “−ω_(n)”.The value Im(−ω_(n)) denoted by the equation (17) is assigned to theimaginary part for the subcarrier having the angular frequency “−ω_(n)”.

[0079] Since the error caused by the digital quadrature modulator 15relates to the calculation time for the imaginary-part signals, thephase error is present while the amplitude error is basically absent.Therefore, the value “λ” is set to “1” while the value “γ” differs from“0”. In the case of QPSK, the amplitudes “A” and “B” are equal to eachother. Thus, the equations (12), (13), (16), and (17) are rewritten asfollows.

Re(+ω_(n))=(½)A{cos α+cos β+cos (α+γ)−cos (β−γ)  (18)

Im(+ω_(n))=(½)A{sin α−sin β+sin (α+γ)+sin (β−γ)  (19)

Re(−ω_(n))=(½)A{cos β+cos α+cos (β−γ)−cos (α+γ)  (20)

Im(−ω_(n))=(½)B{sin β−sin α+sin (β−γ)+sin (α+γ)  (21)

[0080] In the case of QPSK, each of the angles “α” and “β” assumes oneof π/4, 3π/4, 5π/4, and 7π/4. The values Re(+ω_(n)), Im(+ω_(n)),Re(−ω_(n)), and Im(−ω_(n)) expressed by the equations (18)-(21) dependon the angles “α” and “β” as follows.

[0081] When the angles “α” and “β” are equal to π/4, the valuesRe(+ω_(n)), Im(+ω_(n)), Re(−ω_(n)), and Im(−ω_(n)) are equal to “1−sinγ”, “cos γ”, “1+sin γ”, and “cos γ” respectively.

[0082] When the angles “α” and “β” are equal to π/4 and 3π/4respectively, the values Re(+ω_(n)), Im(+ω_(n)), Re(−ω_(n)), andIm(−ω_(n)) are equal to “{square root}{square root over (2)} cos(π/4+γ)”, “{square root}{square root over (2)} sin (π/4+γ)”, “{squareroot}{square root over (2)} cos (3π/4−γ)”, and “{square root}{squareroot over (2)} sin (3π/4−γ)” respectively.

[0083] When the angles “α” and “β” are equal to π/4 and 5π/4respectively, the values Re(+ω_(n)), Im(+ω_(n)), Re(−ω_(n)), andIm(−ω_(n)) are equal to “cos γ”, “1+sin γ”, “−cos γ”, and “−1+sin γ”respectively.

[0084] When the angles “α” and “β” are equal to π/4 and 7π/4respectively, the values Re(+ω_(n)), Im(+ω_(n)), Re(−ω_(n)), andIm(−ω_(n)) are equal to “1”, “1”, “1”, and “−1” respectively.

[0085] When the angles “α” and “β” are equal to 3π/4 and π/4respectively, the values Re(+ω_(n)), Im(+ω_(n)), Re(−ω_(n)), andIm(−ω_(n)) are equal to “{square root}{square root over (2)} cos(3π/4+γ)”, “{square root}{square root over (2)} sin (3π/4+γ)”, “{squareroot}{square root over (2)} cos (π/4−γ)”, and “{square root}{square rootover (2)} sin (π/4−γ)” respectively.

[0086] When the angles “α” and “β” are equal to 3π/4, the valuesRe(+ω_(n)), Im(+ω_(n)), Re(−ω_(n)), and Im(−ω_(n)) are equal to “−1 −sinγ”, “cos γ”, “−1+sin γ”, and “cos γ” respectively.

[0087] When the angles “α” and “β” are equal to 3π/4 and 5π/4respectively, the values Re(+ω_(n)), Im(+ω_(n)), Re(−ω_(n)), andIm(−ω_(n)) are equal to “−1”, “1”, “−1”, and “−1” respectively.

[0088] When the angles “α” and “β” are equal to 3π/4 and 7π/4respectively, the values Re(+ω_(n)), Im(+ω_(n)), Re(−ω_(n)), andIm(−ω_(n)) are equal to “−cos γ”, “1−sin γ”, “cos γ”, and “−1−sin γ”respectively.

[0089] When the angles “α” and “β” are equal to 5π/4 and π/4respectively, the values Re(+ω_(n)), Im(+ω_(n)), Re(−ω_(n)), andIm(−ω_(n)) are equal to “cos γ”, “−1−sin γ”, “cos γ”, and “1−sin γ”respectively.

[0090] When the angles “α” and “β” are equal to 5π/4 and 3π/4respectively, the values Re(+ω_(n)), Im(+ω_(n)), Re(−ω_(n)), andIm(+ω_(n)) are equal to “−1”, “−1”, “−1”, and “1” respectively.

[0091] When the angles “α” and “β” are equal to 5π/4, the valuesRe(+ω_(n)), Im(+ω_(n)), Re(−ω_(n)), and Im(−ω_(n)) are equal to “−1 +sinγ”, “−cos γ”, “−1 −sin γ”, and “−cos γ” respectively.

[0092] When the angles “α” and “β” are equal to 5π/4 and 7π/4respectively, the values Re(+ω_(n)), Im(+ω_(n)), Re(−ω_(n)), andIm(−ω_(n)) are equal to “{square root}{square root over (2)} cos(5π/4+γ)”, “{square root}{square root over (2)} sin (5π/4+γ)”, “{squareroot}{square root over (2)} cos (7π/4−γ)”, and “{square root}{squareroot over (2)} sin (7π/4−γ)” respectively.

[0093] When the angles “α” and “β” are equal to 7π/4 and π/4respectively, the values Re(+ω_(n)), Im(+ω_(n)), Re(−ω_(n)), andIm(−ω_(n)) are equal to “1”, “−1”, “1”, and “1” respectively.

[0094] When the angles “α” and “β” are equal to 7π/4 and 3π/4respectively, the values Re(+ω_(n)), Im(+ω_(n)), Re(−ω_(n)), andIm(−ω_(n)) are equal to “cos γ”, “−1+sin γ”, “−cos γ”, and “1+sin γ”respectively.

[0095] When the angles “α” and “β” are equal to 7π/4 and 5π/4respectively, the values Re(+ω_(n)), Im(+ω_(n)), Re(−ω_(n)), andIm(−ω_(n)) are equal to “{square root}{square root over (2)} cos(7π/4+γ)”, “{square root}{square root over (2)} sin (7π/4+γ)”, “{squareroot}{square root over (2)} cos (5π/4−γ)”, and “{square root}{squareroot over (2)} sin (5π/4−γ)” respectively.

[0096] When the angles “α” and “β” are equal to 7π/4, the valuesRe(+ω_(n)), Im(+ω_(n)), Re(−ω_(n)), and Im(−ω_(n)) are equal to “1+sinγ”, “−cos γ”, “1−sin γ”, and “−cos γ” respectively.

[0097] It should be noted that QPSK may be replaced by BPSK (binaryphase-shift keying) or multi-value QAM. The previously-mentionedcompensation for the modulation error caused by the timing phasedifference can be applied to any modulation system in which a positivesubcarrier frequency and a negative subcarrier frequency are setrelative to a central frequency, and information pieces to betransmitted are assigned to signal points of the positive and negativesubcarriers.

[0098] In the case where both the phase error and the amplitude errorare present so that the value “λ” differs from “1” and the value “γ”differs from “0”, the correction-resultant signal points used by thedata mapping are determined on the basis of the equations (12), (13),(16), and (17). In this case, it is possible to correct both the phaseerror and the amplitude error.

[0099] It should be noted that the data selector 15E in the digitalquadrature modulator 15 may generate the recurrence of “Qn”, “I_(n)”,“−Q_(n)”, and “−I_(n)” or the recurrence of “−Q_(n)”, “−I_(n)”, “Qn”,and “I_(n)” instead of the recurrence of “I_(n)”, “−Q_(n)”, “−I_(n)”,and “Qn”.

What is claimed is:
 1. A method of generating a transmission signal,comprising the steps of: determining non-compensation signal points in atwo-dimensional plane without considering a signal error caused bydigital quadrature modulation, the two-dimensional plane being definedby a real axis and an imaginary axis, the real axis corresponding toreal-part signal components, the imaginary axis corresponding toimaginary-part signal components; determining compensation signal pointsin the two-dimensional plane in response to a signal error caused bydigital quadrature modulation if the non-compensation signal points areused, the non-compensation signal points and the compensation signalpoints being point-symmetry; sequentially assigning digital informationsignal pieces to one of the compensation signal points in response tocontents of the digital information signal pieces; and subjecting thedigital information pieces to a modulation process including digitalquadrature modulation in response to the assignment given by theassigning step to generate a radio-frequency transmission signal.
 2. Amethod as recited in claim 1 , wherein the compensation signal pointsprovide compensation for an error in the radio-frequency transmissionsignal which is caused by one of a phase difference between an in-phasesignal and a quadrature signal, an amplitude difference between thein-phase signal and the quadrature signal, and an error in a quadraturerelation between the in-phase signal and the quadrature signal.
 3. Amethod as recited in claim 1 , wherein the compensation signal pointsprovide compensation for an error in the radio-frequency transmissionsignal which is caused by a timing difference between an in-phase signaland a quadrature signal.
 4. A method of generating a transmissionsignal, comprising the steps of: determining first non-compensationsignal points in a two-dimensional plane without considering a signalerror caused by digital quadrature modulation, the two-dimensional planebeing defined by a real axis and an imaginary axis, the real axiscorresponding to real-part signal components, the imaginary axiscorresponding to imaginary-part signal components; determining secondnon-compensation signal points in the two-dimensional plane withoutconsidering the signal error caused by digital quadrature modulation;determining first compensation signal points in the two-dimensionalplane for a first subcarrier in response to a signal error caused bydigital quadrature modulation if the first non-compensation signalpoints are used, the first non-compensation signal points and the firstcompensation signal points being point-symmetry; determining secondcompensation signal points in the two-dimensional plane for a secondsubcarrier in response to a signal error caused by digital quadraturemodulation if the second non-compensation signal points are used, thesecond non-compensation signal points and the second compensation signalpoints being point-symmetry, the second subcarrier being equal infrequency to the first subcarrier and being different in polarity fromthe first subcarrier; sequentially assigning first digital informationsignal pieces to one of the first compensation signal points in responseto contents of the first digital information signal pieces; sequentiallyassigning second digital information signal pieces to one of the secondcompensation signal points in response to contents of the second digitalinformation signal pieces; and subjecting the first digital informationpieces and the second digital information pieces to a modulation processincluding digital quadrature modulation in response to the assignmentsgiven by the assigning steps to generate a radio-frequency transmissionsignal containing the first and second subcarriers.
 5. An apparatus forgenerating a transmission signal, comprising: means for determiningnon-compensation signal points in a two-dimensional plane withoutconsidering a signal error caused by digital quadrature modulation, thetwo-dimensional plane being defined by a real axis and an imaginaryaxis, the real axis corresponding to real-part signal components, theimaginary axis corresponding to imaginary-part signal components; meansfor determining compensation signal points in the two-dimensional planein response to a signal error caused by digital quadrature modulation ifthe non-compensation signal points are used, the non-compensation signalpoints and the compensation signal points being point-symmetry; meansfor sequentially assigning digital information signal pieces to one ofthe compensation signal points in response to contents of the digitalinformation signal pieces; and means for subjecting the digitalinformation pieces to a modulation process including digital quadraturemodulation in response to the assignment given by the assigning means togenerate a radio-frequency transmission signal.
 6. An apparatus forgenerating a transmission signal, comprising: first means for storinginformation representing assignment of states of a signal piece tosignal points predetermined in response to an estimated signal errorcaused by digital quadrature modulation in the absence of correction;second means for assigning an input information signal piece to one ofthe signal points in response to a state of the input information signalpiece according to the information stored in the first means to convertthe input information signal piece into first and second baseband signalpieces; and third means for subjecting the first and second basebandsignal pieces generated by the second means to a modulation processincluding digital quadrature modulation to convert the first and secondbaseband signal pieces into a piece of a modulation-resultanttransmission signal from which a signal error caused by the digitalquadrature modulation is removed.